The present invention relates to a powderous lithium transition metal oxide, containing a special type of Mn and Ni bearing LiCoO2. The cathode powder can be prepared at large scale by a low-cost process. More specifically, the preparation is the sintering of a mixture of a cobalt containing precursor, like LiCoO2, a Ni—Mn—Co containing precursor, like mixed hydroxide MOOH, and Li2CO3. The sintering temperature is high enough to allow for an exchange of cations between the LiCoO2 and Li—Ni—Mn—Co oxide phases being formed, which results in a very specific morphology with a compositional gradient of the different transition metals. The lithium transition metal oxide powder can be used as a cathode active material in rechargeable lithium batteries.
Despite of some inherent limitations like poor safety and high cost LiCoO2 still is the most applied cathode material for rechargeable lithium batteries. There is a strong demand driven by customer expectation to increase the energy density of rechargeable lithium batteries. One way to improve the energy density is to increase the charge voltage, which requires more robust cathode materials which can be charged at higher voltage. Problems which appear or become more severe if the charging voltage is increased are (a) low safety, (b) poor storage properties during storage of charged batteries at elevated temperature and (c) poor cycling stability. Numerous approaches have been disclosed to address these problems. Partial improvements have been achieved but the basic problems have not been fully resolved.
Beside the demand to increase the energy density, it is essential that rechargeable batteries meet the power requirements. That means that the battery as a whole and particularly the active cathode material itself has a sufficient high rate performance.
There exist general trends. Careful studying of published results on cathode materials allows to better understand the limitations of LiCoO2 based rechargeable lithium batteries.
One basic limitation originates from the surface area dilemma. Increase rate performance (i.e. high power) can be met by increasing the surface area because the solid-state lithium diffusion length can be decreased; which results in an improved rate performance. However, a high surface area increases the area where unwanted side reactions between electrolyte and charged cathode take place. These side reactions are the course of poor safety, poor cycling stability at elevated voltage and of poor storage properties of charged cathode at elevated temperature. Furthermore, high surface area materials tend to have a low packing density which reduces the volumetric energy density.
Another basic limitation originates from the cobalt stoichiometry. Lithium-nickel-manganese-cobalt oxide based cathode materials (like LiMn1/3Ni1/3Co1/3O2) have higher stability against reactions between electrolyte and cathode than LiCoO2, and the raw material cost is lower, but these materials suffer from a lower volumetric energy density and these materials typically have a lower lithium diffusion constant.
It can be concluded that there exist basic limitations in:    Surface area: Low surface area cathode materials are desired to achieve high safety, improved density and high stability during storage; however, the surface area cannot be lowered too much because this will lower the rate performance.    Composition: LiMO2 cathodes, where M dominantly is cobalt is desired to achieve high lithium diffusion rate and high volumetric energy density; however a high content of cobalt causes poor safety properties, increased cost and an inferior high voltage stability.
A solution to this dilemma would be to increase the diffusion constant. Increased D would allow to lower the surface area without losing rate performance.
LiMO2, where M=Ni—Mn—Co with Ni:Mn>1, has been previously disclosed. U.S. Pat. No. 6,040,090 (Sanyo), for example, discloses a wide range of compositions LiMO2 (M=Mn, Ni, Co) including LiMO2 with Ni:Mn>1. The patent application discloses that LiMO2 has a high degree of crystallinity (small HWFM of peaks in the X-ray diffraction pattern). LiCoO2 doped with Ni and Mn has for example been disclosed in U.S. Pat. No. 7,078,128. U.S. Pat. No. 7,078,128 discloses LiCoO2, doped by equal amounts of Ni and Mn is a preferred implementation.
European patent application EP1716609 A1 discloses a LiMO2 based active cathode material where the composition of the particles depends on the size of the particles, particularly, the cobalt content of particles decreases with decreasing size of the particles. The decrease of cobalt content originates from core-shell structured particles, where the Mn—Ni containing shell has the same thickness, covering a LiCoO2 core. As a result, if the particles are small, the LiCoO2 core is small and the cobalt content of the whole particle is low.
European patent application EP1556915 A1 discloses a LiMO2 with a gradient of transition metal composition. The gradient originates from a mixed hydroxide shell, covering the core which has significantly different metal composition. In a preferred implementation the core is LiCoO2. After sintering a gradient of transition metal composition with a radial change of stoichiometry is achieved, and a LiMO2 shell covers a LiCoO2 based core. During sintering, cobalt diffuses from the LiCoO2 core to the LiMO2 shell. At the same time much less Ni diffuses from the LiMO2 shell into the LiCoO2 core. Therefore the shell swells and the LiCoO2 core contracts. A swelling shell covering a shrinking core typically causes the creation of voids between shell and core. These voids are highly undesired.
It is an object of the present invention to define a cathode material having a high rate performance, and showing high stability during extended cycling at high charge voltage. The high temperature storage properties are also improved.